The invention relates to the field of optical couplers, and in particular to optical couplers that couple light between optical fiber and on-chip optical waveguides.
Integrated Optical Circuits (IOCs) have been under development for a number of years. The designers of these circuits seek to integrate several optical functions on a single semiconductor chip. IOCs generally include waveguides formed from several layers of material. For example, in a two-dimensional planar waveguide, a core layer is sandwiched between two layers of clad material.
There is a need to couple light from optical fibers to these on-chip waveguides and vice versa. However, there are a number of other factors that complicate such coupling. First, IOCs are typically high index-contrast systems so as to provide stronger light confinement in smaller dimensions, while optical fibers are low index-contrast systems. That is, there is a much larger refractive index difference between a waveguide's core and cladding than between a fiber's core and cladding. Second, there is a mode diameter mismatch between fibers and waveguides. For example, a fiber may have a core diameter of 8–10 μm, while a single-mode waveguide core may have a width of 0.6–2 μm and thickness of 0.6–2.0 μm. Lastly, there is a mode shape mismatch. Fiber typically maintains a circular mode field, while waveguides typically maintain an elliptical mode field. Thus, as shown in FIG. 1, if a fiber is directly butt-coupled to a waveguide, high power losses occur. In FIG. 1 the fiber is directly butt-coupled to the waveguide at z=4 μm.
Some common fiber-to-chip coupling schemes have arisen in an attempt to overcome such losses. To confine light in the vertical dimension, prior schemes have used GRIN lenses on the fiber-side or high silica graded index stacks on-chip. For example, Shiraishi et al in “Spot size reducer for standard SMF utilizing a graded-index fiber tip,” 11th International Conference Optical Communications- Vol. 1, No. 97- pp 50–3, has demonstrated that a tip with a square-law index profile can be used as a spot size reducer. In this paper, the authors reported a method to reduce the single-mode fiber spot size to 5 μm that uses a quarter-pitch length of a graded-index fiber (GIF).
On the chip side, a planar GRIN lens on silica has been fabricated as described by Beltrami et al in “Planar graded-index (GRIN) PECVD lens,” Electronics Letters-14th 96- Vol. 32; No. 6, pp 549–50. In this paper, the author varies n around 1.44 by adjusting the F doping (CF4 flow in oxide PECVD). The periodic refocusing effect of the parabolic profile is demonstrated. Use of doping to achieve the profile, however, prevents an index of refraction higher than 1.46.
To confine light in the lateral dimension, a laterally tapered waveguide has been used to couple from a photonic device to a single-mode fiber. However, a direct tapered structure has not been used for coupling from a single-mode fiber to a waveguide since there is no practical way to vertically taper an on-chip structure to provide vertical confinement. A lateral taper alone still results in high-coupling loss. One way to solve the problem is a tapered rib waveguide used in conjunction with a much bigger mesa waveguide underneath, as shown by Smith et al in “Reduced coupling loss using a tapered-rib adiabatic-following fiber coupler” IEEE Photonics Technology Letters Vol. 8, No. 8, August 96- pp 1052–4. Light is injected in the mesa, coupling up due to higher n and the tapered ridge. This device still has a size on the millimeter order.
Whatever the precise merits, features and advantages of the above cited references, none of them achieve or fulfills the purposes of the invention.